The invention relates to microwave vacuum tube devices and, in particular, to microscale vacuum tubes (microtubes).
The modem communications industry began with the development of gridded vacuum tube amplifiers. Microwave vacuum tube devices, such as power amplifiers, are essential components of microwave systems including telecommunications, radar, electronic warfare and navigation systems. While semiconductor microwave amplifiers are available, they lack the power capabilities required by most microwave systems. Vacuum tube amplifiers, in contrast, can provide microwave power which is higher by orders of magnitude. The higher power levels are because electrons can travel faster in vacuum with fewer collisions than in semiconductor material. The higher speeds permit larger structures with the same transit time which, in turn, produce greater power output.
In a typical microwave tube device an input signal interacts with a beam of electrons. The output signal is derived from the thus-modulated beam. See, e.g., A. S. Gilmour, Jr., Microwave Tubes, Artech House, 1986, 191-313. Microwave tube devices include gridded tubes (e.g., triodes, tetrodes, pentodes, and klystrodes), klystrons, traveling wave tubes, crossed-field amplifiers and gyrotrons. All contain a cathode structure including a source of electrons for the beam (cathode), an interaction structure (grid or gate), and an output structure (anode). The grid is used to induce or modulate the beam.
Conventional vacuum tube devices are typically fabricated by mechanical assembly of the individual components. The components are made separately and then they are secured on a supporting structure. Unfortunately, such assembly is not efficient or cost-effective, and it inevitably introduces some misalignment and asymmetry into the device. Some attempts to address these problems have led to use of sacrificial layers in a rigid structure, i.e., a structure is rigidly built with layers or regions that are removed in order to expose or free the components of the device. See, e.g., U.S. Pat. No. 5,637,539 and I. Brodie and C. Spindt, xe2x80x9cVacuum microelectronics,xe2x80x9d Advances in Electronics and Electron Physics, Vol. 83 (1992). These rigid structures present improvements, but still encounter formidable fabrication problems.
The usual source of beam electrons is a thermionic emission cathode. The emission cathode is typically formed from tungsten that is either coated with barium or barium oxide, or mixed with thorium oxide. Thermionic emission cathodes must be heated to temperatures around 1000 degrees C. to produce sufficient thermionic electron emission current, e.g., on the order of amperes per square centimeter. The necessity of heating thermionic cathodes to such high temperatures creates several problems. For example, the heating limits the lifetime of the cathodes, introduces warm-up delays, requires bulky auxiliary equipment for cooling, and tends to interfere with high-speed modulation of emission in gridded tubes.
While transistors have been miniaturized to micron scale dimensions, it has been much more difficult to miniaturize reliable vacuum tube devices. This difficulty arises in part because the conventional approach to fabricating vacuum tubes becomes increasingly difficult as component size is reduced. The difficulties are further aggravated because the high temperature thermionic emission cathodes used with conventional vacuum tubes present increasingly serious heat and reliability problems in miniaturized tubes.
A promising new approach to microminiaturizing vacuum tubes is the use of surface micromachining to make microscale triode arrays using cold cathode emitters such as carbon nanotubes. See Bower et al., Applied Physics Letters, Vol. 80, p. 3820 (May 20, 2002). This approach forms tiny hinged cathode, grid and anode structures on a substrate surface and then releases them from the surface to lock into proper positions for a triode.
FIGS. 1A and 1B illustrate the formation of a triode microtube using this approach. FIG. 1(a) shows the microtube components formed on a substrate 1 before release. The components include surface precursors for a cathode 2, a gate 3 and an anode 4, all releasably hinged to the substrate 1. The cathode 2 can comprise carbon nanotube emitters 5 grown on a region of polysilicon. The gate 3 can be a region of polysilicon provided with apertures 6, and the anode 4 can be a third region of polysilicon. The polysilicon regions can be lithographically patterned in a polysilicon film disposed on a silicon substrate. The carbon nanotubes can be grown from patterned catalyst islands in accordance with techniques well known in the art. The high aspect ratio of the nanotubes ( greater than 1000) and their small tip radii of curvature (xcx9c1 to 30 nm), coupled with their high mechanical strength and chemical stability, make them particularly attractive as electron emitters. FIG. 1B shows the components after the release step which is typically manual. Release aligns the gate 3 between the cathode 2 and the anode 4 in triode configuration.
The term xe2x80x9cflexural memberxe2x80x9d includes any structure that induces or allows movement of a structural region into its desired configuration in the device. xe2x80x9cPop-upxe2x80x9d indicates that the structural region is induced to move upon release, without the need for external force. xe2x80x9cHinge mechanismxe2x80x9d indicates one or more flexural members, e.g., a hinge, that allows the component to be moved, e.g., rotated, by applying external force. The cathode structure contains a cathode and one or more grids. The input structure is where the microwave signal to be amplified is introduced (in some configurations, the input structure is a grid of the cathode structure). The interaction structure is where the electron beam interacts with the microwave signal to be amplified. The output structure is where the amplified microwave power is removed, and the collection structure is where the electron beam is collected after the amplified microwave power has been removed.
FIG. 2, which is useful in illustrating a problem to which the present invention is directed, is a scanning electron microphoto which shows an exemplary surface micromachined triode device. On the surface of the device substrate 10, e.g., a silicon nitride surface on a silicon wafer, are formed a cathode electrode 12 attached to the device substrate 10 surface by a hinge mechanism 13 and a spring 11, a grid 14 attached to the device substrate 10 surface by a hinge mechanism 15, and an anode 16 attached to the device substrate 10 by a hinge mechanism 17. Also on the substrate 10 surface are contacts 18 electrically connected to the cathode electrode 12, grid 14, and anode 16. The contacts 18 and connective wiring are typically polysilicon coated with gold, although other materials are possible. Design of the connective wiring should take into account the subsequent rotation of the cathode electrode 12, grid 14, and anode 16, to avoid breakage and/or reliability problems. The substrate 10 also has three locking mechanisms 24, 26, 28, which secure the cathode 12, grid 14, and anode 16 in an upright position, as discussed below. All these components, including the hinges, are formed by a surface micromachining process. The inset is a magnified view of the aligned and patterned carbon nanotubes 19 (deposited on the cathode 12), placed against the MEMS gate electrode (grid 14).
The cathode electrode 12, with attached emitters 19, the grid 14, and the anode 16, are surface micromachined and then mechanically rotated on their hinges, 13, 15, 17 and brought to an upright positionxe2x80x94substantially perpendicular to the surface of the device substrate 10. The locking mechanisms 24, 26, 28 are then rotated on their hinges to secure the cathode electrode 12, grid 14, and anode 16 in these upright positions.
In the structure of FIG. 2, the cathode electrode, the grid, and the anode are arranged such that their surfaces are substantially parallel to each other, and substantially perpendicular to the substrate. Vacuum sealing and packaging of the structure are then effected by conventional techniques.
In operation, a weak microwave signal to be amplified is applied between the grid and the cathode. The signal applied to the grid controls the number of electrons drawn from the cathode. During the positive half of the microwave cycle, more electrons are drawn. During the negative half, fewer electrons are drawn. This modulated beam of electrons passes through the grid and goes to the anode. A small voltage on the grid controls a large amount of current. As this current passes through an external load, it produces a large voltage, and the gridded tube thereby provides gain. Because the spacing between the grid and the cathode can be very small, a microtube triode (or other gridded microtube) can potentially operate at very high frequencies on the order of 1 GHz or more.
The term xe2x80x9cmicrotubexe2x80x9d as used herein refers to a silicon chip supported vacuum tube amplifier for high frequency RF or microwave power wherein the cathode-grid distance is less than about 100 micrometers and preferably less than 20 micrometers. The cathode-anode distance is typical less than 2000 micrometers and preferably less than 2000 micrometers and preferably less than 500 micrometers. The active area of each cathode in a cathode array is typically less than one square micrometer and preferably less than 0.1 square micrometer. The term covers all gridded microtubes including silicon chip supported triodes, tetrodes, pentodes and klystrodes.
While microtube device function has been demonstrated, the field emission efficiency needs further improvements. The intensity and performance of electron field emission are strongly dependent on the electric field applied between the cathode and the gate (grid) and the field between the cathode and the anode. The cathode-gate gap spacing needs to be controlled to a few micrometers. The manual flip-up of the micromachined electrodes into the desired vertical position fails to provide consistent control of the cathode-gate gap spacing, especially if there are inhomogeneities in the height of the nanotube emitters. Accordingly there is a need for improved microtube devices having more precisely controlled electrode spacing and for improved methods for making such devices.
In accordance with the invention, improved vacuum microtube devices are provided with arrangements for tunably spacing the gate and the cathode. Tuning can be effected by using an electrostatic or magnetic actuator to move the gate on a spring or a rail. Advantageously a feedback arrangement can be used to control the spacing. Magnetic reassembly components can be provided for facilitating release of tube components in fabrication.